PARENT SESSION
Symposium #13: Trophic cascades in complex and simple terrestrial systems.

Organized by: MD Moran and WE Snyder
Tuesday, August 6. 1:00 PM to 4:30 PM. Crystal Ballroom, TCC.

Biodiversity cascades: indirect top-predator effects on plant species diversity mediated by herbivore anti-predator behavior.

Schmitz, Oswald^*,1, ¹ Yale University, New Haven

ABSTRACT- Several recent syntheses of direct and indirect interactions in food webs suggest that trophic cascades-the indirect effect of top-predators on plants-should not be a widespread, strong phenomenon in speciose terrestrial systems. This conclusion derives from experimental studies that routinely use total plant biomass in treatments as the response measure. In certain cases, however, individual plant species may have antagonistic abundance responses to experimental manipulations, depending on the way herbivores mediate interspecific plant competition. Such compensatory responses may result in little or no net effect on total plant biomass but can have important implications for plant species composition of the community. I will illustrate, using insights from several years of field experimentation, how hunting spider predators may enhance old-field plant species diversity by altering the way their grasshopper prey utilize those plants. Experiments in this system have shown that in the absence of predators, grasshoppers preferentially feed on nutritionally superior grasses and inflict considerable damage to them. Predator presence causes grasshoppers to forego feeding on grasses and to seek refuge in leafy herbs. In so doing, the grasshoppers preferentially exploit the competitive dominant herb Solidago rugosa. Such preferential feeding releases other herbs from competitive dominance by S. rugosa, thereby enhancing old-field herb diversity with limited alteration of total herb biomass. In this case, cascading effects of top predators on plant diversity appears to be driven by an indirect keystone effect mediated by herbivore anti-predator behavior. The study shows that measuring plant responses to top predator manipulations by pooling plant species, as is done in most studies of trophic cascades, may give the misleading impression that top predators have limited effects in speciose terrestrial communities.

KEY WORDS: trophic cascades, old-field food webs, plant species diversity, herbivore anti-predator behavior